1. Field of the Invention
The present invention relates to a heat treatment apparatus and a heat treatment method for heating a thin plate-like precision electronic substrate such as a semiconductor wafer and a glass substrate for a liquid crystal display device (hereinafter referred to simply as a “substrate”) by irradiating the substrate with flashes of light.
2. Description of the Background Art
In the process of manufacturing a semiconductor device, impurity doping is an essential step for forming a pn junction in a semiconductor wafer. At present, it is common practice to perform impurity doping by an ion implantation process and a subsequent annealing process. The ion implantation process is a technique for causing ions of impurity elements such as boron (B), arsenic (As) and phosphorus (P) to collide against the semiconductor wafer with high acceleration voltage, thereby physically implanting the impurities into the semiconductor wafer. The implanted impurities are activated by the subsequent annealing process. When annealing time in this annealing process is approximately several seconds or longer, the implanted impurities are deeply diffused by heat. This results in a junction depth much greater than a required depth, which might present a problem in good device formation.
In recent years, attention has been given to flash lamp annealing (FLA) that is an annealing technique for heating a semiconductor wafer in an extremely short time. The flash lamp annealing is a heat treatment technique in which xenon flash lamps (the term “flash lamp” as used hereinafter refers to a “xenon flash lamp”) are used to irradiate the surface of a semiconductor wafer with flashes of light, thereby raising the temperature of only the surface of the semiconductor wafer doped with impurities in an extremely short time (several milliseconds or less).
The xenon flash lamps have a spectral distribution of radiation ranging from ultraviolet to near-infrared regions. The wavelength of light emitted from the xenon flash lamps is shorter than that of light emitted from conventional halogen lamps, and approximately coincides with a fundamental absorption band of a silicon semiconductor wafer. Thus, when a semiconductor wafer is irradiated with flashes of light emitted from the xenon flash lamps, the temperature of the semiconductor wafer can be raised rapidly, with only a small amount of light transmitted through the semiconductor wafer. Also, it has turned out that the irradiation of a semiconductor wafer with flashes of light in an extremely short time of several milliseconds or less allows a selective temperature rise only near the surface of the semiconductor wafer. Therefore, the temperature rise in an extremely short time with the xenon flash lamps allows only the activation of impurities to be achieved without deep diffusion of the impurities.
U.S. Patent Application Publication No. US-2005-0063448-A1 discloses a technique in which a light measuring part in a flash lamp annealer includes a calorimeter disposed outside a chamber body, a light guide structure for guiding light emitted inside the chamber body to the calorimeter, and a calculating part for performing computations on the basis of an output from the calorimeter, the light measuring part being adapted to measure the energy of light emitted from flash lamps inside the chamber body by means of the calorimeter. U.S. Patent Application Publication No. US-2005-0063448-A1 also discloses a technique in which the surface temperature of a substrate is obtained by computations, based on the energy of flashes of light measured by means of the calorimeter.
U.S. Patent Application Publications Nos. US-2009-0067823-A1 and US-2009-0103906-A1 present techniques in which an IGBT (insulated gate bipolar transistor) is used to provide chopper control over the current flowing through a flash lamp, thereby causing the flash lamp to emit light for a period of time (approximately 10 milliseconds or more) longer than that for which the flash lamp simply emits light. The use of such techniques allows the surface temperature of a semiconductor wafer to increase or decrease slightly slowly, thereby achieving the better activation of impurities and the recovery of defects created in a layer deeper than an impurity-doped layer during ion implantation. However, the temperature can be said to increase or decrease slowly, as compared with the ultrahigh-speed increase or decrease in temperature caused by simple light emission from the flash lamp, and this increase or decrease in temperature occurs in a very short time as compared with that caused by conventional halogen lamp annealing and the like.
The technique disclosed in U.S. Patent Application Publication No. US-2005-0063448-A1 is to measure the total energy (the total amount of heat) of a single flash of light. The light emission from a flash lamp is caused by discharging electrical charges stored in a capacitor with a predetermined voltage between lamp electrodes. Thus, the total energy of a single flash of light is determined generally by the capacitance of the capacitor and the charging voltage on the condition that all of the electrical charges stored in the capacitor are discharged.
However, in the techniques in which the IGBT is used to provide chopper control over the current flowing through a flash lamp as disclosed in U.S. Patent Application Publications Nos. US-2009-0067823-A1 and US-2009-0103906-A1, the waveform pattern of the emission intensity of the flash lamp is freely adjustable. For example, these techniques are capable of causing the flash lamp to emit high-intensity light for a short period of time (several milliseconds) or to emit relatively low-intensity light for a long period of time (tens of milliseconds). Different intensity waveforms of flashes of light with the same total energy of the flashes of light provide different surface temperature profiles of semiconductor wafers, resulting in different maximum temperatures achieved. For heat treatment using irradiation with light as disclosed in U.S. Patent Application Publications Nos. US-2009-0067823-A1 and US-2009-0103906-A1, it is therefore impossible to determine the surface temperature of a semiconductor wafer from the energy of flashes of light using the technique disclosed in U.S. Patent Application Publication No. US-2005-0063448-A1.
If the chopper control using the IGBT is not provided, there have been cases where different intensity waveforms of light emitted from flash lamps with the same total energy occur due to variations between the flash lamps (e.g., individual differences in distance between electrodes and in sealed-in gas pressure) and the deterioration of the flash lamps themselves. In such cases, a semiconductor wafer has fluctuating maximum temperatures achieved, which in turn makes it impossible to precisely calculate the temperature from the energy of the flashes of light.